Pteroic acid is a convenient starting material for making folate derivatives that can be conjugated to drugs for use as conjugates in therapies that include vitamin receptor-based targeting. Vitamin-drug conjugates can be targeted, for example, to cancer cells that uniquely express, over-express, or preferentially express vitamin receptors. Illustratively, pteroic acid has been used as the starting material for the preparation of a conjugate comprising a folic acid derivative linked to fluorescein via a gamma carboxyl-linked ethylene diamine bridge. This conjugate is described in U.S. patent application Ser. No. 09/822,379, the disclosure of which is incorporated herein by reference. The conjugate is used to label pathogenic cells, such as cancer cells, with fluorescein to make the cancer cells antigenic resulting in their recognition and elimination by the host immune system.
Pteroic acid may be prepared by a variety of conventional means including by synthesis, microbial degradation of folic acid, enzymatic degradation of folic acid, hydrolysis of folic acid, and other conventional methods. Generally, pteroic acid prepared by these methods is contaminated by folio acid, often in substantial amounts. For example, pteroic acid prepared by enzymatic degradation can contain as much as 25% folic acid. Accordingly, efficient methods are needed to remove folic acid contaminants and other impurities from preparations of pteroic acid.
Vitamin-drug conjugates, including conjugates of pteroic acid may be prepared using synthetic methods. In some cases, those synthetic methods may also lead to the formation of side products, impurities, or other contaminants. Accordingly, synthetic and/or purification methods are needed to either avoid the formation of these side products, impurities, or other contaminants, or to remove these side products, impurities, or other contaminants from the vitamin-drug conjugates.